The degradation of proteins by the proteasome is important for protein homeostasis and crucial for a variety of cellular processes. The proteasome, however, consists of a collection of complexes that all have in common the same core particle (CP). This CP can associate with one or two other proteins or protein-complexes, here referred to as CP-interacting factors (CPIFs). As a result there are theoretically 15 differen CP-containing complexes in yeast and even more in humans. Interestingly, some combinations of CP-CPIFs have been observed, while others never seem to form in vivo. This indicates mechanisms have evolved to regulate and coordinate the formation of the different CP- containing complexes. Our lack of knowledge concerning the principles that govern this process complicates efforts to understand the important cellular functions of these different proteasome forms. The long-term goal of this research is to understand the mechanisms that control the formation of the different proteasome complexes in the cell as well as their dynamic exchange. The objective of the research proposed here is to identify the intrinsic controls that drive the association between CP and the different CPIFs and determine their role in vivo. The hypothesis is that CP exists in different physical states that determine the affinity for specific CPIFs. This hypothesis is based on preliminary data that show certain CPIFs bind differently to native CP as compared to CP treated with proteasome inhibitors or immature CP. The yeast Saccharomyces cerevisiae will be used as a model organism, to test this hypothesis by 1) conducting competition assays amongst the different interacting factors complexes. 2) By determining the mechanism and role of CP-allostery in the formation of different proteasome complexes. 3) By determining the function of and mechanism involved in the change of CPIF binding upon CP-maturation. A combination of yeast genetics, in vivo and in vitro techniques will be used to investigate these aims. The expectation is that completion of these aims will provide a fundamental understanding concerning the mechanisms that govern the interaction between CP and CPIFs. Differences in responses amongst the CPIFs can be part of the cellular mechanism in responding to specific cellular stresses (like oxidative stress) and treatments. For the later, for example, the FDA-approved drug bortezomib has been shown to differently affect CP-CPIF interactions when comparing the binding of two CPIFs to CP. Furthermore, a thorough understanding of the formation and regulation of the different proteasome complexes is increasingly important, considering the potential of targeting the proteasome in diseases like neurodegeneration and cancer, as well as in emerging important roles of the non-standard proteasome complexes.